1. Field of the Invention
The subject invention relates generally to a device for monitoring the level of consciousness of a patient under anesthesia, and more particularly, to a device that includes an interface circuit configured to reduce signal noise and radio frequency (RF) interference susceptibility.
2. Description of the Related Art
There is an emerging field for devices used to assist medical personnel in monitoring a patient's response to anesthesia and level of sedation during surgical procedures. The devices are configured to receive and process bio-electric signals from real-time brain activity in patients under sedation. The bio-electric signals are generally low voltage electrical signals in the sub-microvolt range, and are measured using a plurality of physiological sensors (e.g., electrodes) placed on the forehead of the patient. These low voltage signals can be extremely difficult to detect due to signal noise and RF interference that often mask the desired signals.
The device, referred to hereinafter as a monitoring device, includes among other components, an analog interface configured to receive and process the detected bio-electric signals prior to transmitting the resultant signals to an internal processor for further processing. Like any device that houses multiple electrical components in relative close proximity, the monitoring device is susceptible to signal noise and RF interference generated from a variety of sources. One such signal noise source is low-frequency noise, primarily from U.S. line frequencies, which can be capacitively coupled to the patient and to the monitoring device through building infrastructure, power cords, and other patient-connected equipment. RF interference may be presented to the device through radiated or conducted means, and can be generated by equipment in the operating room (such as electrosurgical generators and patient monitoring equipment) or by radio transceivers (such as cell phones, pagers, RF tracking systems). Consequently, signal noise and aliased RF interference may be present in the critical bandwidth of the bio-electric signals being analyzed; thereby adversely affecting the accuracy of the monitoring device.
One known technique for addressing noise and RF interference introduced into the critical bandwidth is common-mode cancellation. For example, U.S. Patent Application No. 2008/0243021 discloses an analog interface that includes a common-mode cancellation circuit that outputs a reference signal for cancelling common-mode voltage present at the electrodes. In other words, the cancellation circuit filters the incoming low voltage electrical signals.
Though common-mode cancellation is an effective technique, there remains an opportunity to provide an improved analog interface that further increases the monitoring device's immunity to signal noise and RF interference.